In　the　simplified　calculation　of　first-order　longitudinal　natural　vibration　period　of　cable-stayed　bridges　with　the　floating　and　tower-girder　hinge　system,　the　influence　of　bidirectional　vibration　coupling　effect　was　neglected.　Taking　the　situation　into　consideration,　simplified　formulas　for　calculating　the　first-order　longitudinal　natural　vibration　period　of　two　kinds　of　cable-stayed　bridges　were　derived　by　the　Rayleigh　energy　method.　Firstly,　simplified　calculation　models　of　cable-stayed　bridges　with　the　floating　and　tower-girder　hinge　system　were　established　respectively,　on　the　basis　of　the　transmission　path　of　longitudinal　inertial　force　under　the　action　of　earthquake.　Secondly,　in　the　light　of　assumed　longitudinal　and　vertical　displacement　vibration　equations　of　main　girder　and　tower　of　cable-stayed　bridges　with　floating　and　tower-girder　hinge　system,　the　deformation　energy　and　kinetic　energy　of　cables,　main　girder　and　tower　of　two　kinds　of　cable-stayed　bridges　were　analyzed　and　calculated　respectively,　considering　the　intercoupling　between　the　longitudinal　and　the　vertical　modes.　Finally,　simplified　formulas　of　first-order　longitudinal　natural　vibration　period　for　cable-stayed　bridges　with　the　floating　and　tower-girder　hinge　system　were　derived　by　energy　conservation　principle.　The　first-order　longitudinal　modal　of　five　built-up　bridges　were　analyzed　by　dint　of　the　finite　element　software.　Then　theoretical　results　were　compared　with　values　calculated　by　the　finite　element　method.　The　results　show　that　results　calculated　by　simplified　formulas　are　in　good　agreement　with　values　calculated　by　the　finite　element　method.　And　the　relative　errors　are　less　than　15%.　The　simplified　calculation　method　with　good　stability　can　be　used　to　estimate　the　first-order　longitudinal　natural　vibration　period.　It　provides　a　reference　for　preliminary　design　and　scheme　comparison　of　cable-stayed　bridges　by　combing　the　simplified　formulas　for　calculating　the　first-order　longitudinal　natural　vibration　period　of　cable-stayed　bridges　with　floating　system　with　ones　with　tower-girder　hinge　system.　©　2017,　Editorial　Department　of　China　Journal　of　Highway　and　Transport.　All　right　reserved.